Contrary to popular belief, the hydropneumatic suspension system used in Citroën vehicles is remarkably reliable — provided it is properly maintained. The main cause of problems is not the complex design but neglected maintenance and errors when topping up the fluid. A mechanic familiar with the LHM system can keep it running for decades.
How the hydropneumatic system works
The system replaces conventional springs and dampers with hydropneumatic spheres — metal spheres divided by a flexible membrane. On one side of the membrane is nitrogen under pressure; on the other is LHM fluid. The main pump (belt-driven from the engine) generates working pressure of around 140–170 bar, which passes through the central distributor to the spheres and suspension struts.
The height corrector — a mechanical or electrohydraulic device at each axle — continuously levels the vehicle regardless of load. This is what gives Citroën vehicles their unique ride comfort: the car does not squat under the weight of luggage or passengers.
System variants — passive and Hydractive
Passive systems (CX, BX) — classic hydropneumatics without electronics. Suspension stiffness is fixed and mechanically set via valves.
Hydractive 1 (XM from 1987) — the first electronically controlled variant. The ECU switches an additional sphere at each wheel, changing the suspension characteristic between comfort and sport modes. Sensors: speed, throttle position, braking.
Hydractive 2 (Xantia, later XM) — an improved version with faster switching and more sensors. The Xantia Activa adds active hydraulic anti-roll bars eliminating body roll in corners — a unique solution in automotive history.
Models equipped with the LHM system
- Citroën CX (1974–1991) — classic passive hydropneumatics
- Citroën BX (1982–1994) — simplified version with MacPherson struts at the front; a softer rear than front ride is a design characteristic
- Citroën XM (1989–2000) — Hydractive 1 (to approx. 1993) and Hydractive 2
- Citroën Xantia (1992–2001) — Hydractive 2; Activa version with active anti-roll bars
Classic LHM models: DS (1955–1975), SM (1970–1975), GS/GSA (1970–1986).
Note: The Citroën C5 (X3, from 2001) and C5 II (X7) and C6 use the synthetic LDS fluid — not LHM. Always identify the suspension variant before topping up.
LHM fluid — properties and absolute restrictions
LHM (Liquide Hydraulique Minéral) is a green mineral hydraulic fluid developed by Total for Citroën. Key properties:
- Colour: intense, vivid green — this signals good condition. Brown, dark or contaminated fluid must be replaced
- Operating temperature: wide range, does not freeze at typical European temperatures
- Composition: mineral — incompatible with synthetic seals
Critical warning: Never mix LHM with other hydraulic fluids: ATF, DOT, and above all with the synthetic LDS from newer Citroëns. Any foreign fluid in the LHM system destroys the rubber seals within minutes of operation. Replacing all seals, distributors and struts costs more than the value of many of these vehicles.
Fluid level should be checked with a cold engine and the vehicle in the lowered (parking) position (lever down). The LHM reservoir is usually on the right side of the engine bay.
LHM filter in the reservoir must be cleaned at every fluid change. A blocked filter restricts flow and accelerates pump wear.
Fluid replacement — recommended every 5–7 years or 60,000–100,000 km. Old LHM loses its lubricating and anti-corrosion properties, accelerating pump and valve wear. Replacement in a Hydractive system requires flushing all circuits and is more labour-intensive than in passive systems.
Hydropneumatic spheres — the heart of the system
The LHM system uses several types of sphere:
- Suspension spheres — one per wheel (or two at the front on some models), the main springing element
- Anti-roll bar spheres — in the Xantia Activa and some XMs, stabilising the body in corners
- Pressure accumulator — buffers working pressure, allowing the system to operate briefly after the engine is switched off
The sphere membrane gradually loses elasticity or nitrogen diffuses through the membrane — the sphere becomes "dead". All spheres age, so when buying a used LHM vehicle, sphere replacement is the first item on the inspection list.
Sphere test without tools:
- Warm the engine to operating temperature
- Press firmly on the bumper several times (front or rear)
- Good sphere: 2–3 clear rebounds, vehicle returns to position smoothly
- Dead sphere: no rebound or vehicle returns immediately without damping
Common problems and diagnosis
Firm ride
Diagnose in sequence:
- Sphere test — as above
- LHM colour and level
- LHM filter condition
- Height corrector settings
- Wheel alignment and tyre pressures
It is worth noting: tyres have a huge effect on Citroën ride comfort. Soft radial tyres (e.g. Michelin Energy or Comfort) can dramatically improve the driving experience.
Suspension dropping quickly after switching off
If the vehicle drops noticeably within a few minutes of switching off the engine, check:
- Pressure accumulator — test as described above (service position after switching off): the vehicle should rise. Dead accumulator = no rise at all
- Distributor valves — a faulty valve allows fluid to return to the reservoir. The rear suspension is particularly susceptible through the brake distributor valve
- Strut leaks — internal leaks are not visible from outside; a leaking strut does not hold pressure
The problem of rear suspension dropping quickly in the Xantia and XM is very common and does not require immediate repair if the vehicle rises normally after the engine is started.
Pump running almost continuously
Continuous pump operation means a leak in the system — the pump is compensating for lost pressure rather than maintaining it. Check:
- Dead pressure accumulator (most common cause)
- Visible LHM leaks under the vehicle and in the engine bay
- Leaking high-pressure pipe fittings
- Worn suspension struts (internal leak)
Continuous pump operation with a healthy system shortens its service life — do not delay diagnosis.
Vehicle does not rise after starting
Failure to rise after starting indicates:
- Serious wear or failure of the main pump
- Blocked pressure regulator valve
- Exceptionally large system leaks
- Completely empty LHM reservoir
In any of these cases an immediate workshop visit is required — driving with failed hydropneumatic suspension causes further damage and can compromise safety.
Problems specific to Hydractive (XM, Xantia)
Hydractive 1 and 2 systems have additional solenoid valves controlled by the ECU. A failed valve or sensor causes the system to operate permanently in emergency mode — usually in the firm setting. Diagnosis requires a PSA tool (Lexia/Diagbox) to read suspension ECU fault codes.
A characteristic symptom of Hydractive failure: the vehicle does not change suspension stiffness with speed or throttle input — it always rides identically hard or soft.
Step-by-step diagnostic procedure
- LHM level and colour — visual inspection of the reservoir
- Sphere test — for each axle separately
- Accumulator test — switch off engine, move lever to high position, vehicle should rise
- Leak inspection — engine bay, under vehicle, at struts
- LHM filter — check for blockage
- Height correction — does the vehicle sit level and does the corrector respond to load changes
- Hydractive (if applicable) — read suspension ECU fault codes
Note: Do not tow an LHM vehicle without first securing the suspension. After the engine is switched off and the accumulator is exhausted, the vehicle drops to minimum suspension height. Sudden jolts when towing in this position can damage the struts. Inform the recovery company of the vehicle's specific requirements.
Workshop tools and services
If you're not sure which spheres fit your vehicle, use our interactive I.F.H.S. sphere catalogue — find the right spheres by model, engine and year of manufacture.
We also offer suspension sphere regeneration, LHM pump rebuilding and hydraulic parts supply. We regenerate spheres for H I and H II systems (Hydractive 1 and 2 — LHM). Contact us for details and a quote.
